The invention concerns a switching element for a valve train of an internal combustion engine for switching to different valve lifts, said switching element comprising an outer element having an inner element arranged for axial displacement therein, each of the outer and the inner element comprising a reception aligned to each other in a relative position, at least one piston being arranged in at least one of the receptions for sliding toward the other of the receptions to couple the inner element to the outer element in said relative position, and a high-position stop for defining said relative position being arranged between the inner and the outer element.
A switching element of the pre-cited type is disclosed in the generic prior art document DE 199 15 531 which shows a switchable cam follower for a tappet push rod drive. The high-position stop for defining the relative position is formed by a piston-like element arranged in the inner element. This piston-like element projects radially outward into a longitudinal groove of the outer element. In the axially extracted state of the inner element from the outer element, the piston-like element abuts against one end of the longitudinal groove. The purpose of this is to create an aligned arrangement of a coupling bore made in the outer element for a piston arranged in the inner element for achieving coupling.
A drawback of this prior art is that an adjustment of lash in the coupling is relatively complicated and cost-intensive. It is clear that the reception (coupling bore) in the outer element for receiving the piston must be configured with a slight lash relative to the outer peripheral surface of the piston. This lash and the high position differ from switching element to switching element due to fabrication conditions. The relatively wide range of variance of this mechanical idle travel in switching elements is, however, undesirable.
In other words, a lash, i.e. an idle travel that the inner element undergoes relative to the outer element upon coupling and loading by the cam till the outer element participates in the force flow from the cam can be compensated by an appropriate dimensioning of the cam contour. In practice, however, this results in a relatively large dispersion of lash. This leads to an undesired variance of the overlap of the intake and exhaust valves. Besides this, an undesired large amount of wear occurs if the lash between the coupling piston and its surrounding reception is too large.
To keep the coupling lash or its variance within acceptable limits, the prior art has recourse to a grouping of the coupling pistons. This procedure is extremely expensive from the point of view of fabrication and measuring techniques. For example, the switching elements are first completely mounted, the lash is then measured following which, the switching elements are dismounted again and an appropriate piston is then chosen for coupling. It is equally conceivable to group the high-position stops provided on the longitudinal grooves of the outer elements.
It is an object of the invention to provide a switching element of the pre-cited type in which the aforesaid drawbacks are eliminated by simple measures.
This and other objects and advantages of the invention will become obvious from the following detailed description.
The invention achieves the above objects by the fact that at least one part of the high-position stop is configured as an adjustable, separate element which in a particularly preferred embodiment of the invention, is a piston-like element, a first section of whose outer periphery is fixed in a recess of one of the inner element and the outer element, and a second section of whose outer periphery engages a guide such as a slot that is made in the other of the inner element and the outer element and has a length that is at least equal to a relative motion between the inner element and the outer element. One end of the guide forms a further part of the high-position stop, and at least that end of the piston-like element that is situated opposite said one end of the guide comprises a chamfer that bears against a complementary chamfer of the said one end of the guide in the relative position, said piston-like element being axially displaceable in the relative position for establishing an alignment of the receptions of the inner and the outer element.
Due to the fact that the piston-like element is axially displaceable in its recess and, in the ideal case but not necessarily, has a conical surface forming the high-position stop, the aforesaid drawbacks are eliminated by simple measures During the assembly of the piston-like element to form the high-position stop (pressing or screwing-in or any other method familiar to the person skilled in the art) the mechanical idle travel of the coupling element (piston) relative to its surrounding reception can be adjusted by choosing different assembling depths. This means in other words, that in the axially extracted state of the inner element from the outer element, the high-position stop of the invention enables the axis of the reception for effecting coupling to be adjusted or brought into alignment to the axis of the reception for the piston.
The scope of this invention extends explicitly not only to switching elements such as cam followers in tappet push rod drives or cup tappets and support elements for finger levers but also to switching elements installed directly in lever-type cam followers. The scope of this extension extends equally to other mechanical locking devices that are installed, for instance, in camshaft adjusters and the like for adjusting their start position.
In a particularly simple variant of an embodiment of a peripheral section of the piston-like element that cooperates with the guide, this section has a generally conical configuration as described above. This section or the entire outer peripheral surface can also be precision worked by centerless grinding or the like
The receptions for the piston and the recess for the piston-like element extend preferably radially through the switching element that preferably has a cylindrical shape. If the piston is arranged in the inner element, it is particularly advantageous from the fabrication point of view if the recess of the inner element for fixing the piston-like element is aligned to the reception for the piston in the inner element. Advantageously, in this case, a through-bore can be made.
According to further features of the invention when used in a series of switching elements for internal combustion engines of the same type, a lash of the pistons relative to the surrounding receptions, when coupled, is the same in all the switching elements in their unloaded state, and a distance of the pistons from stops of the surrounding receptions, which stops are situated axially opposite the ends of the guides in the coupled state, is also the same in all the switching elements in their unloaded state. This applies, of course, only in the ideal case but the invention enables the variance of the lash of the elements to be kept within such narrow limits that there is no, or no noteworthy influence on the valve overlap mentioned above. The order of magnitude of this lash is 0.1 mm or approximately this value, with a tolerance of +/xe2x88x920.010 mm. Thus, generally speaking, the lash or the idle travel is adjusted in the range of one tenth of a millimeter and the tolerance is in the range of one hundredth of a millimeter.
The invention will now be described more closely with reference to the appended drawing.